It is known in the art to package chemical agents in water soluble unit dose packages, whereby a package is thrown into water so that the contents disperse in the water as the packaging film dissolves. Water soluble unit dose packaging is advantageous in applications which employ hazardous chemicals as are found in, for example, agrochemicals, bleaching agents, laundry detergents, industrial chemicals, pool chemicals, and the like. Water soluble packaging allows a user to employ the hazardous substances without coming into direct contact with dangerous chemicals. Additionally, unit dose packaging obviates the need for the user to measure the chemicals since each dose is pre-packaged.
Polyvinyl alcohol (PVOH) films have been used in the art to form unit dose packages, because the films exhibit good strength, impact resistance, and are soluble in water. Unfortunately, however, the solubility of PVOH films rapidly decreases when they are exposed to certain chemicals such as oxidizing chemicals, acid chemicals, alkali chemicals, chlorine-containing substances, salts with polyvalent metals, boric acid, polyamines, insecticides, herbicides, among others. Thus, in applications which employ these types of chemicals, the use of PVOH packaging is impractical as the effective shelf-lives of the unit dose products are limited by their solubility in water.
Various approaches to improve the chemical resistance of PVOH films are described in the art. U.S. Pat. No. 6,956,070 to Fujiwara et al. entitled “Water-Soluble Film and Package Using the Same,” describes a water-soluble polymer film for use as unit dose packaging where the film includes a polyvinyl alcohol copolymer which contains N-vinylamide monomer units in amounts of from 1 to 10 mol percent. The PVOH copolymer may further include carboxyl functional groups or alkali metal salts thereof.
U.S. Pat. No. 6,608,121 to Isozaki et al. entitled “Water-Soluble Resin Composition and Water-Soluble Film” discloses a PVOH based film for unit dose packaging, where the PVOH resin is formulated with a plasticizer and a starch component. The films in Isozaki et al. are reported to exhibit good physical properties and have acceptable water solubility after prolonged exposure to alkali substances.
U.S. Pat. No. 6,166,117 to Miyazaki entitled “Water-Soluble Film” relates to a film for packaging chemical substances, where the film includes a polyvinyl alcohol copolymer which contains sulfonic acid functional monomer units, which are typically acrylamido-2-methylpropanesulfonate (AMPS) units. The film further includes gallic acid or salts of gallic acid. According to Miyazaki, the water-solubility of the films does not diminish with prolonged exposure to acidic chemicals, chlorine-containing chemicals, or agricultural chemicals.
A similar approach is disclosed in Japanese Abstract 7-118407 to Miyazaki entitled “Agricultural Packaging Film” which discloses water-soluble unit dose packaging which includes a polyvinyl alcohol resin that is modified with sulfonic acid containing monomers. The sulfonic acid units are preferably AMPS monomers or salts thereof, and may be included in the copolymer in amounts of from 0.1 to 20 mole percent.
U.S. Pat. No. 6,787,512 to Verrall et al. entitled “Water-Soluble Copolymer Film Packet,” relates to a water-soluble film which includes a vinyl alcohol copolymer. The copolymers in Verrall et al. may include either carboxylate functional units or sulfonate functional units, and the films are stated to be useful as unit dose packaging for liquid laundry detergents.
The use of sulfonic acid modified PVOH films for packaging hypochlorite salts and harsh acids is described in copending U.S. application Ser. No. 11/147,910 to Vicari entitled “Vinyl Alcohol Copolymers for Use in Aqueous Dispersions and Melt Extruded Articles” (US Patent Application Publication No. 2005/0222355). According to that application, the modified PVOH copolymer is generally produced according to a unique process which allows for a high content of polymerized AMPS in the polymer backbone.
Other references of interest include U.S. Pat. Nos. 6,821,590 and 7,005,168, both to Verrall et al.
While the above noted approaches provide improved films for use with certain chemicals, the present Applicants have discovered that many of the modified films still exhibit a rapid reduction in solubility when exposed to aggressive oxidizing agents, and in particular chemicals that are useful for chlorinating water such as chlorinated isocyanurates. When exposed to these types of compounds, most PVOH film formulations (including those with PVOH copolymers) show a significant decline in water solubility, in some cases becoming effectively insoluble in less than a week. This is unacceptable for most unit dosing applications, where shelf-lifes of two months or more may be desired. Accordingly, there exists a need for a polymeric film which has excellent water solubility even after prolonged exposure to aggressive oxidizing agents. Further, the film should exhibit good strength properties and should resist discoloration over time.